CXCR4 and breast cancer: IL-6 has been shown to promote breast cancer metastasis by upregulating C-X-C chemokine receptor type 4 (CXCR4) through c-Jun, STAT3, and nuclear factor kappa B (NF-κB) [136,137], and facilitate angiogenesis through STAT3 by upregulating vascular endothelial growth factor (VEGF), matrix metallopeptidase 9 (MMP9), and basic fibroblast growth factor (bFGF) in the tumor microenvironment [138,139].